Social media has long been a key battlefield in the so-called war on terror.

But the US has found this year that online ads on social media websites like Facebook, rather than posts, are a cost-effective way to fight the propaganda of the Islamic State (IS) and other militant groups.

Michael Lumpkin, the State Department's point man in the "soft power" fight against militant information, said on Thursday at the Defense One Summit in Washington that Facebook's detailed metrics for advertisers helps the government campaign reach its targets - people who might be groomed online by militants.

"Using Facebook ads, I can go within Facebook, I can grab an audience. I can pick country X, I need age group 13 to 34, I need people who liked Abu Bakr al-Baghdadi or any other set, and I can shoot and hit them directly with messages," he said.

"In some places in the world, it's literally pennies a click to do it," he said.

Lumpkin is the Coordinator of the Global Engagement Center at the State Department, a unit focused on both conducting its own campaigns against militants and coordinating with other groups to the same end, especially in on-line and mobile spaces.

He said the detailed user data that social media giants collect is crucial in identifying his target audience, on the social media that they prefer.

"That's why these Facebook ads that I mentioned are kind of a fundamental change for us: for very low investment we can be highly targeted towards the enemy."

Facebook, he noted, offers the government access to affordable amassed and collated user data for singling out target groups and individuals for anti-militant ads the US government runs.

"The best I can do right now is to have access to big data and to use the analytics tools on the social media platforms, the Facebooks and the others," he said.